The present invention is directed to an apparatus and method for obtaining measurement of the magnitude of ultrasonic random backscatter from tissue forming a region of distributed, unresolved ultrasonic reflectors. Such tissue may comprise that of the myocardium and differs from tissue forming a more highly resolved reflector, such as the mitral valve. More particularly, the present invention is directed to an apparatus and method for obtaining the optimal measurement of ultrasonic random backscatter magnitude from such tissue.
The optimal magnitude measurement is the maximum likelihood or minimum variance estimate of the magnitude.
The myocardium is the muscular wall of the heart. Through its contraction and relaxation, the heart is driven to pump blood through the circulatory system. Constrictions or obstructions in the blood vessels serving the heart muscle can result in a myocardial infarction or "heart attack" in which a volume of tissue is permanently injured by the circulatory loss. However, prior to infarction there may be a far greater tissue volume in which blood supply is already deficient. In this volume, the tissue has been damaged but the possibility exists for reversing, or at least stabilizing, the damage by appropriate rest, drug therapy, or surgery. Such tissue is termed "ischemic." With the recognition that myocardial infarction is a dynamic process, extensive efforts have been directed to protecting ischemic myocardial tissue in an effort to avoid or reduce infarction.
These efforts have been hampered by a lack of means to accurately and reliably determine the location and volume of ischemic tissue in the myocardium in order to quantify the injury.
Electrocardiography is of assistance in determining if heart muscle damage has occurred and the magnitude of the damage but is of limited assistance in mapping ischemic myocardial tissue. Angiography, in which a radio-opaque dye is injected in the coronory arteries can determine the location and extent of obstruction, but again, mapping of ischemic muscle is left somewhat to inference. There is also a medical risk to an already sick patient associated with angiography, particularly in the catheterization required to inject the dye.
As the result, other techniques have been sought to provide an accurate, reliable determination of the location and volume of ischemic muscle while reducing patient risk. Ultrasonic imaging presents one possibility. Ultrasonic imaging is a non-invasive and non-injurious diagnostic technique in which acoustic energy is applied to the body by an ultrasonic transducer. The returning backscatter or echo signals are received by the transducer, recorded, and analyzed. Because of its non-invasive nature and high level of safety, ultrasound has found considerable use in diagnostic procedures, as for example, viewing a fetus in utero or scanning the brain or breast for pathological conditions.
With respect to the application of ultrasound to cardiology, extensive use of echocardiography has been made in analyzing the operation of the mitral valve of the heart. The difference in the acoustic impedance between the blood filled heart chambers and the tissue of the valve forms a resolvable reflector for the ultrasonic energy. This makes for a distinct backscatter signal that is relatively easy to process and interpret.
The myocardium is considerably more difficult to image ultrasonically, since its tissue comprises a region containing, non-resolving reflectors or targets for the acoustical energy. Such a region has multiple, fine reflecting structures that produce multiple, fine echos that are individually unresolvable. Further, the individual elements of the fine, reflecting structure are spatially distributed. The returning signal is from a region rather than a well defined anatomical feature. Mathematical techniques must be utilized to deal with the random backscatter from such a region. Optimal techniques for ultrasonically examining the myocardium do not currently exist.
Certain ultrasonic random backscatter properties of myocardial tissue are known, as are the alterations in the random backscatter properties when the myocardial tissue is ischemic. Such properties and alterations include the following. First, the backscatter signal is amplitude modulated in accordance with the contractions of the myocardium. Specifically, the magnitude of the backscatter signal is greater when the myocardium is relaxed and decreases when the heart muscle contracts. Second, the magnitude of the amplitude modulation changes with ischemia. Specifically, the magnitude of the amplitude modulation becomes less with ischemia, i.e. there is a smaller difference between the peaks and valleys of the modulation. Also, there is a phase shift in the amplitude modulation with respect to the cardiac cycle with ischemia. Third, the magnitude of the backscatter signal, time averaged over a heart beat, is increased with ischemia.
The foregoing phenomena have been determined invasively as with the ultrasonic transducer applied directly to the myocardium. However, it can be readily appreciated that direct application of an ultrasonic imaging transducer to the heart muscle is ordinarily precluded from the medical and practical standpoints so that it is not possible to map ischemic myocardial tissue using its backscatter properties with this technique.
What is needed is a technique by which the magnitude of ultrasonic backscatter from the myocardium and its amplitude modulation and phase characteristics can be accurately obtained non-invasively, i.e. with the ultrasonic transducer applied to the external wall of the chest, rather than directly on the heart.
However, it will be appreciated that noninvasive imaging of the myocardium is considerably more difficult than direct, invasive imaging for a number of reasons. The parts of the body through which the ultrasonic signals must pass from the transducer on the chest to the myocardium in the thorax, and back, have a deleterious effect, often termed bulk tissue loss, on the echo signals received by the transducer. This is in addition to the effects of various aspects of the instrumentation, including the magnitude and frequency of the signal transmitted by the transducer, the frequency responses of the transducer and circuitry of the instrumentation, and the relationship of the transducer and target tissue, termed diffraction. The poor ultrasonic properties of the myocardium, described above, further complicates matters.
As a result, existing ultrasonic apparatus and methods fail to provide backscatter data from myocardium containing signal characteristics that are medically useful, for example, in diagnosing and mapping myocardial ischemia.
While the diagnosis and mapping of ischemia through the use of ultrasound has been discussed above, it will be appreciated that a technique for obtaining an optimal magnitude measurement would permit diagnosis of other cardiac conditions ultrasonically. For example, since amplitude modulation of the backscatter is due to the contraction of the heart muscle, close analysis of modulation data could allow determination of the state of contractility of the myocardium. Further, it would be highly desirable to use ultrasound to determine the condition of other tissue besides that of the heart, that also forms an unresolved ultrasonic reflector. Such tissue might include glandular organs, such as the liver or pancreas.